Several techniques are used in the prior art to control a power FET or IGBT used in the upper, or high-side, position of a well known “totem-pole” power circuit. If a P-channel device be used at a relatively large positive voltage, or an N-channel device for a negative voltage, a semiconductor level shifter may be used to control a current which can be dropped across a resistor to enhance the switch. This method does not require a high-side enhancement supply, but is relatively slow, limiting its utility to power-converters operating below a few tens of KHz. Sometimes opto-couplers are employed to convey control signals to high-side switches, but these usually require a floating high-side supply and also are limited in speed and timing integrity. Transformers are often used to convey ground-referred control signals to high-side switches. With these, good timing integrity can be obtained, but resetting their magnetizing currents incurs either the penalty of significant recovery time or of handling high flyback voltages. It is possible to use a relatively small transformer to convey pulses representing signal transitions from ground level to a high-side switch, which allows magnetic recovery. However, doing so requires extra circuitry to reconstruct such pulses to the original control waveform and drive circuitry for the switch, in addition to a high-side power supply for said circuitry. High-side switches may be capacitively driven from ground referred signals. If the switches be of the correct polarity to allow the high-side pole of the capacitor to be referred to the upper supply, and sufficient time and means for resetting the capacitor be provided, this method can provide good timing integrity and does not require a floating supply. However the time and/or the circuitry required to reset the capacitor is often an unacceptable burden. Moreover, such a totem-pole requires complementary switch polarities. P-channel devices are generally either less conductive or slower than equivalent N-channel devices, limiting the desirability of the complementary totem-pole. Capacitive driving of non-complementary totem poles usually impresses large common-mode transient voltages across the coupling capacitor, making such drive complex, or even impractical.
U.S. patent application Ser. No. 12/334,692 teaches a charge-retaining switch that may be directly driven by relatively short pulses conveyed to the power device by one or more transformers. The combination of short pulses and the inherent bi-stability of the switch allow the transformer/s quickly to reset. Thus excellent timing integrity may be obtained in the presence of large high-side voltages without a high-side gate-driver supply, and without complex circuitry between the transformer/s and the high-side switch.
U.S. patent application Ser. No. 12/361,074 teaches a DC common-mode level shifter comprising one or more common-mode chokes spanning the common-mode voltage, and being reset to avoid large common-mode currents. In this application, a differential signal floating at a high-common voltage is shown being referred to ground.